1. Field of the Invention
The present invention relates generally to electronics devices, and more particularly to waveform synthesizers for the generation of voiced musical tones.
2. Description of the Prior Art
Musical instruments each have characteristic voices that relate to the sound amplitude envelope attack rate, decay rate, sustain level and release rate. For example, the envelope of piano sounds has a sharp attack rate when the keys are pressed and then gradually diminishes as the key is held down. By pressing a piano's sustain pedal, the sound will fade away slower. A piano also has a large number of harmonic overtones during the attack. These harmonic overtones decrease over time until a near constant harmonic content results.
Frequency modulation (FM) techniques are conventionally employed in audio sound synthesis digital integrated circuits to generate complex waveforms that contain high harmonic and non harmonic sounds. The correspondence between the modulation index and spectrum distribution of higher harmonics is natural and makes possible a wide range of sound generation. FM sound generation uses effects obtained from modulating a carrier. The pitch, tone and level of a musical sound can be obtained by skillful manipulation of basic FM parameters, e.g., carrier output level, modulator output level, feedback level of the modulator, frequency of the carrier and frequency of the modulator.
John Chowning, U.S. Pat. No. 4,018,121, issued Apr. 19, 1977, describes basic FM synthesis for musical sounds, e.g., a modulating sine wave, a time varying envelope function for the modulating waveform, a carrier sine wave, and a time varying envelope function for the amplitude of the frequency modulated wave. The amplitude of the modulating wave, or the envelope function for the modulating wave, varies with time so that the frequency spectrum of the resulting frequency modulated waveform varies during the attack, sustain, and decay of the sound. It is conventional to use a envelope function to vary the amplitude of the final sound, but Chowning added an envelope function to the modulating waveform to time vary the frequency spectrum of the generated sound.
Music synthesizers commonly use multiple building blocks, "operators", to generate a waveform and execute some function such as amplitude or frequency modulation. The sound of one musical instrument (one channel of the synthesizer) is created by using one or more operators to generate the instrument's characteristic wave shape. Such a wave shape is described as the "tone color" of the instrument. Some instruments can be synthesized more easily than others. The most easily synthesized instruments require only one operator.
Conventional hardware operators are relatively costly. But because the speed of such hardware is relatively very quick compared to the signals they must generate, it is common practice to implement only one or two operators in hardware and then process multiple operators by time division multiplexing within the required sample rate time. The number of perceived operators possible is limited by the sample rate divided by the single hardware operator logic speed, and is therefore fixed for each given synthesizer design.
In general, two operators are required to produce one instrument voice. Each operator consists of a phase modulator and an envelope generator. One operator is typically needed for one percussion sound, although the bass drum sound can require two. A typical synthesizer chip on a card, e.g., the SOUNDBLASTER, has eighteen operators, for a total of eleven voices.
For either FM or AM synthesis, the initial waveform type, e.g. sine wave, triangle wave, square wave, used for either the modulating waveform or the carrier waveform will change the resulting sound. The resulting waveform shape represents what is called the "tone color" or "timbre" perception of the sound. A sine wave is the simplest mathematical waveform input. All other waveforms can be described as being a combination of multiple sine waves. A triangle wave for example can be broken down to a combination of the fundamental sine wave plus a series of the odd harmonics. Each musical instrument sound to be synthesized has its own characteristic waveform shape that can be shown as a combination of sine waves. It is thus desirable in a music synthesizer to have a variety of input waveforms to choose from for generating the sounds in order to match the characteristics of the instrument to be synthesized.
Norio Tomisawa describes, in U.S. Pat. No. 4,249,447, issued Feb. 10, 1981, a method for producing electronic musical instrument tones. Tomisawa discloses an arithmetic unit comprised of an adder and a sinusoid wave memory read by an output "y" of the adder. One input of the adder is connected to an input variable "x" and the other input is connected to an output "sin y" of the sinusoid wave memory after multiplication in a multiplier by a feedback ratio determined by a feedback parameter ".beta.". A product ".beta. sin y" and the variable "x" are therefore input to the adder which produces "x+.beta. sin y", a physical address input for the sinusoidal wave memory. The variable "x" is produced, for example, from circuitry connected to a musical keyboard key. A musical tone is created by passing the output "sin y" through a multiplier connected to an envelope generator. The feedback parameter ".beta." can be changed to control the tone contents of the tone wave output by the arithmetic unit. The technique of using feedback is therefore critical to the operation of the method and devices described by Tomisawa.
Yamaha LSI, Systems Technology Division (San Jose, Calif.), markets a 24-pin integrated circuit (IC) sound generator under the trademark, OPLII, for FM operator type-LII. This IC has three voicing modes selectable by software control. Nine sound simultaneous voicing, six melody, five rhythm and a speech synthesis mode are all possible. A vibrato oscillator and an amplitude modulation oscillator are also included. The shape of the waveform envelope for music, modulation factor, frequency, voicing, mode and other parameters are determined according to data written to registers within the IC. Such data can be combined to generate the sound of a piano, violin or other musical instrument. A very large number of highly complex combinations can be programmed.